Thanks for all the information, everyone. I'm of the opinion now that a small amount of preload is desirable, which rules out the Misumi Standard Grade. The HiWin sets are specified as "Z0 - very light preload" with (apparently) 0µm clearance. The Misumi High Grade are listed as "-3 - 0 µm" clearance.
Based on experience posted here, I would think either is fine for typical printers. I actually have been very pleased with the stainless RobotDigg rails & guides I used on my delta. However, my RailCore is worth the extra money 😃